Infectious diseases still afflict hundred of millions of mammalian and non-mammalian animal species, resulting in physical disabilities and death. In particular, the intensive method of raising animals for food under modern agricultural conditions make these animals extremely susceptible to these diseases. As a result, active immunization against the causative agents of these infectious diseases is required in order to reduce the economical losses associated with them. An ideal vaccine should elicit strong and long-lasting protective immunity with only a few injections, evoke minimal side-effects and should be safe. In the past many vaccines have been developed and successfully applied, although new outbreaks of diseases caused by new agents or by more virulent isolates of existing agents frequently occur. Vaccines can, in principle, be divided into two groups, i.e. live (attenuated) vaccines and inactivated vaccines. The advantages of live vaccines include the presentation of all the relevant immunogenic determinants of an infectious agent in its natural form to the host's immune system, and the necessity of a relatively small amount of the immunising agent, because of its inherent property to multiply in the vaccinated host. A major disadvantage of a live vaccine is concerned with its safety: a live vaccine may induce disease (in immuno-compromised) animals, or the live microorganism may even revert to virulence, as a result of which animals experience a virulent infection. Moreover, with respect to some infectious agents, until now, no attenuated forms of these agents are available.
The disadvantage concerning the safety aspect is not displayed by inactivated vaccines and, hence, constitutes the major advantage over live vaccines. However, a major disadvantage of inactivated vaccines is represented by their intrinsicly low immunogenicity, i.e. inactivated immunogens as such have a limited ability to trigger the host's immune system. Therefore, appropriate means are necessary to augment the immunogenicity of these inactivated immunogens. This type of vaccines normally requires adjuvants with significant immunostimulatory capabilities to reach a minimum potential in preventing disease. However, the desirability of additional immunostimulators for use in combination with inactivated immunogens in order to augment their inherently low immunogenicity is evident, in particular immunostimulators which are applicable to more than one immunogen.
In the field of poultry vaccines U.K. patent no. 2170708 (1986) discloses the preparation of a live vaccine comprising a live attenuated form of a poultry pathogen, such as Newcastle disease virus (NDV) and fowl-pox virus (FPV) which is mixed with a previously prepared water-in-oil (w/o) adjuvant emulsion containing no immunogens, in order to benefit from the advantages of both a live vaccine and a w/o emulsion adjuvanted inactivated vaccine.
European patent application no. 92202864.2 (publication no. EP 0533294), incorporated herein by reference, discloses a poultry vaccine for combating chicken anaemia virus (CAV) infection in poultry, the vaccine being based on an attenuated form of CAV. Furthermore, combination vaccines derived from this attenuated CAV and other poultry pathogens are disclosed.